The invention relates to an apparatus for filtering and separating particularly biologically organic fluids by reverse osmosis and by micro-, ultra-, and nano- filtration. The apparatus comprises a plurality of spaced filter elements with large surface areas which are in contact with a fluid and which include a plurality of separate stacks of filter elements arranged adjacent one another in such a way that the fluid flows through the stacks in series or in a parallel flow relationship. The permeate collected by the filter elements enters draining bolts extending through the stacks of filter elements and is conducted out of the apparatus through permeate discharge conduits.
Such an apparatus is known for example from EP-A-0 707 884. The apparatus disclosed therein includes a housing in which the stacks of filter elements are arranged in series. The housing includes an inlet for the fluids to be separated, an outlet for the retentate and an outlet for the permeate, which is collected by the filter elements and discharged by way of the permeate outlet for further utilization. Within the housing, in which the stacks are arranged, there is provided an essentially open channel for the fluid flow. The particular arrangement of the stacks of filter elements permits very high flow speeds of the fluid between the inlet for the fluid entering the apparatus and the outlet for the fluid leaving the apparatus as retentate. As a result, the formation of deposits for example in the form of filter cakes on the filter elements is prevented since the stacks of filter elements have essentially no areas which are not subject to the fast fluid flow. The fluid can flow from the inlet to the outlet of the apparatus in a straight flow path without any deflections.
Communal as well as commercial waste waters have generally a high content of biologically organic compounds. Such waste waters are said to have a high bio-mass concentration, which can be controlled only by large apparatus and/or procedural expenses if the liquid solvent, generally water, is to be removed from such fluids including biologically organic compounds. The collection of the aqueous solvents may have the purpose of concentrating the fluid so that it can be decomposed in an optimal way and/or the collection of the aqueous medium from the fluid, for the production of useable water and even drinking water from the fluids including the biologically organic compounds.
Such apparatus must be functional even when the fluid is almost stagnant that is even if the fluid moves only at very low speed. The fluid may be in such a state for example in wastewater settling tanks or in open bodies of water, but also in bio-reactors. In those cases, the high flow speeds of the fluid as they exist between the fluid inlet and the outlet for the retentate of the prior art apparatus, which provides for a continuous problem-free operation of the apparatus, are not available.
In many cases, the energy requirements for a continuous undisturbed operation of such an apparatus should also be very low as such apparatus should be operable independently of existing power supply nets.
In that case, high fluid pressures and fluid flow speeds as they are generated in the prior art apparatus by electric circulating pumps cannot be provided.
It is consequently the object of the present invention to provide an apparatus of the type referred to initially, which is capable of safely separating fluids with a high content of biologically organic and/or inorganic compounds. The apparatus should be capable of performing the separation even at very low fluid flow speeds, or even at stagnation and the build up of deposits forming a filter cake on the filter elements should be avoided as much as possible. The apparatus furthermore should be operable efficiently and continuously with low energy consumption and without the need for servicing.
In an apparatus for filtering and separating fluids by reverse osmosis or by micro-, ultra-, and nano filtration, including a plurality of spaced planar filter elements arranged in stacks in spaced relationship to permit their surfaces to be contacted by the fluids for the collection of permeate from the fluids within the filter elements, at least one draining rod having a central draining passage extends through the stack of filter elements and has: openings disposed in each filter element for collecting the permeate therefrom and a vacuum generating means is connected to a permeate discharge which is in communication with the draining rod passage for generating a vacuum in each filter element.
The advantage of the invention resides mainly in the fact that the apparatus may simply be immersedxe2x80x94without housingxe2x80x94into the fluid, for example, into a settling tank, a waste water treatment container, an open body of water, or even a bio-reactor. Only a suitable vacuum has to be generated at the permeate discharge for example by a vacuum pump providing a vacuum of 0.5 to 0.9 bar. However, the elimination of a need for a housing for the apparatus is not the only advantage. It is also possible to effect the separation essentially exclusively by the vacuum generated at the permeate side that is by providing the optimum pressure difference between the fluid and the permeate, which is obtained by providing a suitable pressure difference between the fluid flow medium and the permeate. It is furthermore advantageous that the apparatus provides for an extremely large membrane surface area in a relatively small space and no housing parameters need to be taken into consideration. If the membrane surface area of the apparatus turns out to be insufficient to achieve the desired degree of concentration for the retentate or the desired degree of purity of the permeate the membrane surface area can be increased in a simple manner by adding stacks of filter elements to the apparatus. The apparatus should therefore be so designed that such addition is easily possible at any time.
In a preferred embodiment of the apparatus according to the invention, the filter elements are in the form of membrane pillows as they are generally known in the membrane separation field. It is pointed out however, that the membranes do not need to be in the form of membrane pillows. It is also possible to employ filter elements which consist of a single membrane layer if it is made sure by the design that the side of the membrane element opposite to the fluid to be separated is effectively sealed with respect to the fluid flow so that the fluid cannot mix with the permeate.
Each membrane pillow preferably includes a form-stabilizing frame structure forming a support structure and extending adjacent the outer circumference of the filter element. However, the filter elements, be they in the form of single-sided membrane elements or in the form of membrane pillows, could also be stabilized by external means which prevent the membranes from being pressed together into contact with each other during operation of the apparatus.
Without support means, the effective membrane surface area could be limited and the separation performance could be reduced. Furthermore, the selective separation layers of the membrane elements could be damaged and the membrane elements could become attached to one another if the membranes of adjacent elements would come into contact with each other. This would provide locations for the collection of deposits, which could result in a complete blocking of the filter elements. It is also possible that the fluid includes coarse materials or a large amount of solids which, with mechanically unstable filter elements, could also cause the filter elements to come into contact with one another with the same results as mentioned above. It is therefore advantageous to arrange form stabilizing support elements in the membrane pillows, which extend between the outer support frame elements. If only a single-sided membrane structure is used a stabilizing structure may be disposed above or below the single membrane to provide for a filter element with high individual stability. Such an arrangement preferably has no outer spacer elements as they are known from the state of the art so that the fluid can reach any part of a filter element without any restrictions. This is particularly advantageous if the filter element is used for stagnant or slow-moving fluids.
The filter elements themselves consist preferably of a polymer or a polymer mixture. In that case, they are called polymer membranes. Polymer membranes are known for use in connection with various separating tasks with different chemical compositions and are selected depending on the respective separating task.
In addition to filter elements of polymer or polymer mixtures, filter elements of ceramic material may be provided, which are suitable for special separation tasks which cannot be solved by polymer membranes or which are difficult to solve thereby. This is particularly true for separation tasks with fluids at a temperature at which the polymer and polymer mixture membranes are no longer stable.
The stabilization element itself preferably consists of a material, which is sufficiently temperature and pressure resistant such that the filter element, which includes the stabilization element on the inside or on the outside, is not or only slightly deformed.
In order to improve the discharge of the permeate, that is to reduce the energy requirements at the permeate side, the stabilization element consists preferably of a porous material. Then, the permeate may reach the permeate discharge passages of the apparatus with lower energy losses since the porous material facilitates flowing of the permeate from the permeate side of the membrane.
Preferably, the stabilization element consists of a plastic and/or a metallic and/or a ceramic material or a combination of these materials. Of course, the material for the stabilization element is selected depending on its compatibility with the fluid and on the expected temperature conditions.
It has been said earlier that the apparatus is also designed taking into consideration simplicity and design features which can be realized relatively inexpensively thereby providing advantages when the apparatus is utilized in large numbers and the ease of manufacturing and servicing. In this regard, it is advantageous if a plurality of draining rods or bolts are arranged distributed around the circumference of a stack and the draining bolts include openings spaced along the length of the draining bolt for receiving the permeate and conducting it out of the apparatus through an draining passage extending axially through the draining bolt. Such an arrangement permits the use of standard tubes, which are commercially available, for example, tubular materials of plastic or glass fiber reinforced or of metallic materials. In this way, the bolts may be provided with simple discharge grooves extending axially along the draining bolts. These grooves may include spaced radial openings extending to the interior of the tubes whereby, at one hand, the mechanical stability of the draining bolts is preserved and, on the other hand, a uniform permeate discharge capability is provided.
A disc cutter of small diameter can easily form the axial grooves and the openings.
Preferably, the filter elements of a stack are spaced from one another such that the filter elements do not contact one another during operation of the apparatus. The disadvantages such contacting would have, have been described earlier.
Preferably, the spacing between the filter elements is provided by annular spacer elements. This type of spacing is preferably chosen if the filter elements or filter element stacks are penetrated only by a single draining bolt. However, annular spacer elements may be provided also if two draining bolts penetrate a stack for example. But in that case, strip-like spacer elements are preferred which include openings disposed at the distance of the bores for receiving the two bolts, which, in this case, are spaced, with one of the bolts extending through each opening.
Whether the spacer elements are annular or strip-like, they can be so formed that they do not only fulfill a spacing function but also a sealing function with respect to the permeate discharge space. In this way, the number of parts required for forming a stack of filter elements can be reduced.
The stacks of filter elements may be held together by straps arranged at opposite sides of the stack in spaced relationship. Preferably, spacers are disposed between the straps, which interconnect the straps and hold them at a predetermined distance from each other. In this way, a uniform axial pressure is applied to the stacks of filter elements, whereby the stacks with the spacer elements and possible separate seals are pressure sealed.
The design described above for the support and pressure seal arrangement for the filter elements or respectively, the stack of filter elements provides for a simple but highly effective holding and mounting arrangement which has proven itself in actual use in waste water settling containers and in flowing bodies of water.
The invention will be described below on the basis of the accompanying schematic drawings showing a particular embodiment of the invention.